In the current communication environment, two or more heterogeneous networks may exist. For example, various heterogeneous networks such as a WiMAX network which is an example of a mobile communication system and a WiFi network may exist. The heterogeneous network means a network that uses a communication system (or radio access system) different from that used by a specific network, and a heterogeneous user equipment means a user equipment which belongs to a heterogeneous network that uses a communication system different from that of the specific network.
For example, on the basis of the WiMAX network and the user equipment which belongs to the WiMAX network, since the WiFi network uses a communication system different from that of the WiMAX network, the WiFi network corresponds to a heterogeneous network. The user equipment which belongs to the WiFi network corresponds to the heterogeneous user equipment. By contrast, on the basis of the WiFi network, the WiMAX network may be the heterogeneous network, and the user equipment which belongs to the WiMAX network may be the heterogeneous network.
Also, a multi-mode user equipment means a user equipment that supports two or more wireless access systems or wireless communication systems. WiFi means a short distance communication network (LAN) that allows ultra-speed Internet within a given distance of a place where an access point (AP) is provided, and uses a radio wave or infrared transmission mode and may be referred to as a wireless LAN.
In a wireless environment, a user equipment may use a heterogeneous network which exists to support a heterogeneous user equipment in addition to a radio access technology (RAT) provided to efficiently transmit and receive a signal or improve throughput. At this time, the multi-mode user equipment that supports a plurality of RATs to use two or more heterogeneous networks may transmit and receive a signal without being restricted to a specific RAT by using another RAT that may provide the best service under the current status. The number of heterogeneous networks (a plurality of RATs) to which the multi-mode user equipment accesses to transmit and receive a signal may be two or more. Accordingly, the multi-mode user equipment may transmit and receive a signal to and from each of a base station, which uses RAT different from that of a serving base station, and base stations, which use heterogeneous networks (heterogeneous RAT), or through cooperation between the base stations. Hereinafter, in the present invention, the multi-mode user equipment, which supports two or more wireless access systems or wireless communication systems, will be referred to as “user equipment”.
In the wireless communication system, a cellular network and a wireless LAN coexist, and organized cooperation between the cellular network and the wireless LAN will be required. However, since the cellular network and the wireless LAN have their respective features different from each other, it is preferable to perform mutual complementary operation. However, a method for organized cooperation between the cellular network and the wireless LAN has not been discussed.